Image magnification of as much as 20%–35% is the major disadvantage of aphakic spectacles. Contact lenses magnify images by only 7%–12%, whereas IOLs magnify images by 4% or less. An IOL implanted in the posterior chamber produces less image magnification than does an IOL in the anterior chamber. The issue of magnification is further complicated by the correction of residual postsurgical refractive errors. A Galilean telescope effect is created when spectacles are worn over pseudophakic eyes. Clinically, each diopter of spectacle overcorrection at a vertex of 12 mm causes a 2% magnification or minification (for plus or minus lenses, respectively). Thus, a pseudophakic patient with a posterior chamber IOL and a residual refractive error of −1 D would have 2% magnification from the IOL and 2% minification from the spectacle lens, resulting in little change in image size.
Aniseikonia is defined as a difference in image size between the 2 eyes and can cause disturbances in stereopsis. Generally, a person can tolerate spherical aniseikonia of 5%–8%. In clinical practice, aniseikonia is rarely a significant problem; however, it should be considered in patients with unexplained vision symptoms.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.